Fan Airflow and Power Calculator
Calculates fan power consumption from blade swept area, rotational speed, and static pressure. Use it when sizing ventilation fans or verifying energy budgets for HVAC and industrial systems.
Last updated: September 2026
Formula below · 2 sources (asme.org, Wikipedia) · Updated Sep 2026
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About this calculator
A fan's airflow cannot be calculated exactly from its diameter and speed alone — it depends on blade pitch and the system resistance, and is read from the manufacturer's fan curve. This calculator gives a rough estimate by assuming the air advances about 0.1016 m (4 inches) through the swept disc per revolution, typical of low-pitch axial fans: Q (m³/s) = A × (n / 60) × 0.1016, with swept area A = π × (D/2)² (D converted from mm to m). Airflow in m³/h is Q × 3,600. The air power needed to move that flow against the static pressure is P = Q × ΔP (W). Shaft power is higher by the fan efficiency (often 40–70% for small axial fans), and motor input higher again. Fan laws: flow scales with speed, pressure with speed², power with speed³ — doubling the RPM multiplies power by eight. Use the manufacturer's curve for any real selection.
How to use
Consider a fan with a diameter of 600 mm, spinning at 1,200 RPM against a static pressure of 50 Pa. Swept area: A = π × (0.3)² ≈ 0.2827 m². Estimated flow: Q = 0.2827 × (1,200/60) × 0.1016 = 0.574 m³/s ≈ 2,068 m³/h. Air power: 0.574 × 50 ≈ 29 W. If the fan is 50% efficient and the motor 85% efficient, the electrical input is about 29 / (0.5 × 0.85) ≈ 68 W. The defaults (500 mm, 1,450 RPM, 250 Pa) give about 1,736 m³/h and 121 W.
Frequently asked questions
How does fan diameter affect airflow and power consumption?
Airflow is directly proportional to the swept area, which scales with the square of the diameter. Doubling the diameter quadruples the swept area and, at the same RPM, quadruples the flow rate. Power also increases with area, so a larger fan moves far more air at the same rotational speed. This is why large, slow-spinning fans are more energy-efficient than small, fast-spinning fans for high-flow, low-pressure applications—they achieve the same flow with much less turbulence and noise.
What is static pressure and why does it matter when sizing a ventilation fan?
Static pressure is the resistance the fan must overcome to push air through the duct system, filters, grilles, and any other obstructions. It is measured in Pascals and represents stored pressure energy rather than velocity energy. A fan with insufficient static pressure capability will deliver far less flow than its free-air rating suggests once installed in a real duct system. When sizing a fan, you must calculate the total system resistance at the desired flow rate and select a fan whose pressure-flow (P-Q) curve intersects the system curve at the required operating point.
What are fan affinity laws and how do they help with energy savings?
The fan affinity laws describe how flow, pressure, and power change when fan speed changes. Flow Q is proportional to speed N, static pressure ΔP is proportional to N², and shaft power P is proportional to N³. This cubic relationship with speed is extremely important for energy savings: reducing fan speed by just 20 % cuts power consumption by nearly 50 %. Variable-speed drives (VSDs) exploit this by matching fan speed to actual demand rather than running at full speed with a damper, and they typically pay back their installation cost within one to three years in commercial HVAC applications.